1. Field of the Invention
The invention relates to an all-solid secondary battery that is excellent in reducing an overpotential, reducing the possibility that an electrode active material layer expands and contracts, and reducing the possibility that a conductive path and a metal ion conduction path are interrupted.
2. Description of the Related Art
In a secondary battery, a portion of chemical energy is converted to electric energy due to a chemical reaction. Thus, electricity is discharged from the secondary battery. In addition, when current flows in a direction opposite to a direction in which current flows at the time of electric discharge, electric energy is converted to chemical energy, and stored in the secondary battery (that is, the secondary battery is charged). Among secondary batteries, a lithium secondary battery is widely employed as power sources of, for example, a notebook-sized personal computer and a cellular phone, because the lithium secondary battery has high energy density.
In the lithium secondary battery, when graphite (C6) is used as a negative electrode active material, a reaction represented by a formula (1) proceeds at a negative electrode at the time of electric discharge.C6Li→C6+Li++e−  (1)Electrons generated in the formula (1) flow through an external circuit, and perform work on an external load, and then, reaches a positive electrode. Lithium ions (Li+) generated in the formula (1) move in an electrolyte held between the negative electrode and the positive electrode, from the negative electrode to the positive electrode due to electro-osmosis.
When lithium cobaltate (Li0.4CoO2) is used as the positive electrode active material, a reaction represented by a formula (2) proceeds at the positive electrode at the time of electric discharge.Li0.4CoO2+0.6Li++0.6e−→LiCoO2  (2)At the time of electric charge, a reverse reaction opposite to the reaction represented by the formula (1) proceeds at the negative electrode, and a reverse reaction opposite to the reaction represented by the formula (2) proceeds at the positive electrode. At the negative electrode, the graphite (C6Li), into which the lithium ions have moved due to graphite intercalation, is recovered. At the positive electrode, lithium cobaltate (Li0.4CoO2) is recovered. Thus, the lithium secondary battery is able to discharge.
In the lithium secondary battery, the electrode, into which the electrolyte has been mixed, is generally used to increase mobility of the lithium ions. Particularly, when using the electrode into which a solid electrolyte has been mixed, the electrolyte does not permeate pores in the electrode, and therefore, an ion conduction path, through which ions move, may be interrupted depending on a mixing ratio between the solid electrolyte and the active material in the electrode. Accordingly, the impedance of the electrode may increase. As a result, it may become difficult to charge the secondary battery with large current, and to discharge large current from the secondary battery, and thus, the rate of utilization of the active material may decrease.
Technologies for avoiding the above-described situation when employing the solid electrolyte have been developed. Japanese Patent Application Publication No. 8-195219 (JP-A-8-195219) describes an all-solid lithium secondary battery that includes a positive electrode, a negative electrode, and a solid electrolyte. In the all-solid lithium secondary battery, a mixture, which is produced by mixing active material powder with a given average particle diameter and solid electrolyte powder with a given average particle diameter at a certain weight ratio, is used as at least one of the electrodes.
As shown by the above-described formulae (1) and (2), in the lithium secondary battery, the lithium ions move between the positive electrode and the negative electrode due to electric charge and electric discharge. Conventionally, the positive electrode and the negative electrode alternately expand and contract, because the lithium ions move into, and move out of the positive electrode and the negative electrode. As a result, the battery may be warped or deformed, or a crack may occur in the battery. For example, Japanese Patent Application Publication No. 2009-146657 (JP-A-2009-146657) describes a technology for avoiding the above-described situation, that is, a solid electrolyte lithium secondary battery in which a positive electrode, a solid electrolyte layer, and a negative electrode current collector are sequentially stacked. In the positive electrode, a positive electrode composite layer, which contains positive electrode active material powder and solid electrolyte powder, is formed on each of both surfaces of a positive electrode current collector in the form of a flat plate.
In the technology described in the publication No. 8-195219, the mixing ratio between the active material and the solid electrolyte is optimized. However, no consideration is given to the distribution of the mobility of the ions in the electrode. Also, in the solid electrolyte lithium secondary battery described in the publication No. 2009-146657, a space is actually provided in advance so that the secondary battery can be expanded at the time of electric charge, as shown in FIG. 1 of the publication. Therefore, the technology cannot radically reduce the possibility that, for example, the battery is warped.